The present invention relates to a device for the electric monitoring of the level of a liquid contained in a container, having at least one temperature-dependent resistance probe immersed in the liquid the level of which is to be monitored, a source of constant current which can be connected to the resistance probe, which can be connected to the resistance probe under the control of a program control device means controlling a device for detecting and storing the initial voltage drop on the resistance probe at an initial time and the measurement voltage at a specified time after connection of the source of constant current; and means for evaluating said voltages by subtracting the measurement voltage from the initial voltage and under circumstances inverting the difference to form a display voltage which corresponds to the level.
One such known device (DE-OS No. 27 40 289) proceeds from the basis that the level of the liquid contained in a container can be measured by measuring the change in the resistance of a metallic resistance element as a function of the temperature which it assumes with a given flow of current. The mean temperature of the resistance probe depends on what portion of its total length is immersed in the liquid since the immersed section is cooled more strongly than the section present in the air. The total resistance of the resistance probe therefore depends on the level of the liquid. In order to make the result of the measurement more independent of variations in the surrounding temperature, the mean resistance which the resistance probe assumes upon the passage of current was referred to a mean resistance at an initial time when the resistance probe was not yet heated by the current. Specifically, the device comprises a source of constant current for the resistance probe from the terminals of which source the initial voltage is obtained at an initial time and the measurement voltage at a defined later time. The initial voltage is stored in a storage. When at said defined later time, the measurement voltage is present, the difference between the measurement voltage and the initial voltage is determined in a subtraction circuit. The difference can then be compared with given preestablished voltages which correspond to different levels of immersion of the resistance probe in order to produce a display or an alarm depending on the result of the comparison. This device, which operates in purely analog fashion, has the disadvantage that it is very dependent on the specific properties of the fluid to be measured, particularly an admixture of methanol in gasoline, which effects the thermal conductivity.
It is therefore already known, instead of using a single resistance probe which covers the entire measurement range within the container, to divide this measurement range up by individual resistance probes which are arranged at staggered heights within the container. The means for evaluating the voltage drops on the resistance probes, namely the initial voltage and the measurement voltage, comprise comparators which determine whether the corresponding resistance probe is or is not immersed in liquid. Depending on the result of the comparison, a predetermined signal is given off which corresponds to the height of the highest resistance probe which is immersed. With this incrementally operating level-measuring instrument by which a digital display can be effected, the error remains, in principle, less than the distance between two measureable level stops so that here the accuracy of the measurement can be improved as compared with purely analog measurement. The last-mentioned incrementally operating device, however, has the disadvantage that intermediate values between the individual levels are not determined so that in each case an error equal to the distance between levels must be expected. This error can be reduced in the manner that the increments are selected small which, however, means a large expense for apparatus since a correspondingly large number of resistance probes and corresponding elements of the control and evaluation devices must be provided.